The present invention relates to a method for calibrating a pressure sensor in a fuel metering system as well as to a device for implementing the method, a control element equipped with the device, and a fuel metering system.
Conventional methods and devices exist for calibrating a pressure sensor of a fuel metering system of an internal combustion engine. A fuel metering system may be equipped with a high-pressure pump for transporting fuel from a low-pressure region to a high-pressure region, with injectors, which are controllable as a function of performance quantities, for metering and injecting fuel into the combustion chambers of the internal combustion engine, as well as with at least one pressure sensor for measuring the pressure in the high-pressure region and/or low-pressure region. Fuel metering systems are known, e.g. as so-called common-rail direct fuel-injection systems.
These systems are equipped with a presupply pump and a demand-controlled high-pressure pump. For example, an electric fuel pump, which transports the fuel from a fuel reservoir to the low-pressure region of the system, is used as the presupply pump. In the low-pressure region, there is an admission pressure of about 4 bar. The high-pressure pump transports the fuel from the low-pressure region to a high-pressure accumulator of the system. A significantly higher pressure prevails there, namely a pressure of about 150 to 200 bar in the case of gasoline and a pressure of about 1500 to 2000 bar in the case of diesel fuel. A plurality of injectors, which, in response to being accordingly activated, inject the fuel from the high-pressure accumulator into the combustion chambers of the internal combustion engine at the injection pressure in the high-pressure accumulator, branch off from the high-pressure accumulator. The injectors are controllable as a function of certain operating parameters. Situated in the high-pressure accumulator is a pressure sensor, a so-called rail pressure sensor, which is used to determine the injection pressure prevailing in the high-pressure accumulator and is then used to direct an appropriate electrical signal to a control unit of the internal combustion engine. A pressure control line branches off from the high-pressure region and leads via a pressure control valve into the low-pressure region. A pressure sensor, a so-called presupply pressure sensor, may also be provided there. A low-pressure line branches off from the low-pressure region and leads via a low-pressure regulator back into the fuel reservoir.
Pressure sensors in general, as well as the pressure sensors in the abovementioned fuel metering systems, have a static offset error, i.e., the zero point is not reliably indicated. However, as a result of an offset error, the measured value of the pressure sensors, in particular the measured value acquired by the pressure sensors in the low-pressure region, may deviate significantly from the actual pressure value.
In the starting phase of direct injection common-rail internal combustion engines, there is typically a low pressure. The internal combustion engine is usually started with a low admission pressure generated by the presupply pump and is not switched to the high pressure until later. Since the fuel quantity injected into the combustion chambers by the injectors is particularly dependent on the injection pressure prevailing in the high-pressure accumulator, this injection pressure should be included in the calculation of the injection time in the starting phase of the internal combustion engine. However, this is usually not possible due to the above-described inaccuracies of the pressure sensors. The method for calibrating a pressure sensor described in German Published Patent Application No. 195 47 647 confronts this problem by using a reference pressure to calibrate the pressure sensor prior to starting the internal combustion engine. In this instance, the atmospheric pressure may be used, i.e., the ambient pressure prevailing in the system at a standstill and prior to the start of the internal combustion engine. Therefore, a method and a device for calibrating at least one pressure sensor of a fuel metering system of an internal combustion engine are described in German Published Patent Application No. 195 47 647, where the fuel is transported by a pump from a low-pressure region to a high-pressure region and is metered from there by injectors that are controllable as a function of operating parameters into the combustion chambers of the internal combustion engine, the pressure in the high-pressure region and/or in the low-pressure region being measured by the at least one pressure sensor while the internal combustion engine is in operation, and the atmospheric pressure being measured by the pressure sensor prior to the start of the internal combustion engine in order to calibrate the pressure sensor.
However, the conventional method and the conventional device only function properly when the system is already at atmospheric pressure while calibrating the pressure sensors. For this purpose, it may be required to ensure that the internal combustion engine is not operated during a certain standstill time prior to calibration, so that the pressure in the system is able to decrease and to adjust itself to the ambient pressure level.
An object of the present invention is to propose a method of the species recited at the outset and a corresponding device, which enable the pressure sensor to be calibrated as precisely as possible. This may be achieved in that the cooling-water temperature of the internal combustion engine is measured and the drop in the cooling-water temperature is derived therefrom as a measure for the standstill time of the internal combustion engine, and in that the pressure sensor is first calibrated when the standstill time exceeds a predefinable minimum.
Thus, an arrangment for monitoring the cooling-water temperature already present per se in the vehicle may be used to reliably and precisely calibrate the pressure sensor. Therefore, the present invention is able to be realized very quickly and almost without extra expenses, in particular without using additional timing supervision for measuring the standstill time. Such an exemplary method according to the present invention and the corresponding exemplary device are well suited for calibrating pressure sensors in the high-pressure region (rail pressure sensors) as well as for calibrating sensors in the low-pressure region (presupply pressure sensors).
Accordingly, it may be particularly advantageous when a temperature difference indicating the drop in the cooling-water temperature is determined in that the instantaneous cooling-water temperature is compared to a stored cooling-water temperature previously measured when stopping the internal combustion engine, and the pressure sensor is first calibrated when the temperature difference exceeds a minimum temperature difference corresponding to the predefined minimum. In this context, it may be particularly advantageous when the pressure sensor is calibrated immediately after the control unit of the fuel metering system is initialized. As a result of these measures, the cooling-water temperature only needs to be measured twice, only the cooling-water temperature measured when stopping the engine needing to be stored temporarily until it is compared to the temperature present shortly after the start of the engine.
A particular advantage may also result when the pressure sensor is calibrated in that an atmospheric pressure measured by the pressure sensor during standstill of the internal combustion engine is compared to the absolute value of the atmospheric pressure, the difference between the measured atmospheric pressure and the absolute value indicating a calibration value, which is later applied to the pressure values measured when the internal combustion engine is in operation. The pressure sensor in a diesel rail system has a resolution of measurement of approximately 2 bar. Since the drift may be up to 20 bar, a calibration using 1 bar abs (absolute pressure) is sufficient. However, this is not the case for sensors having a resolution of 1 bar to approximately 6 bar. In this instance, a calibration using the exact atmospheric pressure is desirable since values of 0.01 bar are already important.
In this connection, it may be advantageous when the calibration value is stored in a memory of the control element of the fuel metering system as a stored value until a new calibration value is determined. Therefore, a compensation value for calibrating the pressure sensor is always available.